1. Field of the Invention
This invention relates in general to communication devices and in particular to communication devices having active equalization circuits for audio processing.
2. Description of the Related Art
Communication devices, such as portable cellular or cordless telephones and wired landline telephones, are well known in the art. Voice communication devices typically include a microphone, an earpiece, and a keypad. Recently, many communication devices also include a display. The microphone converts an acoustic input signal into an electric input signal. The earpiece converts an electric output signal into an acoustic output signal. Typically, a person positions the communication device against the person""s head to align the microphone with the person""s mouth and to align the earpiece with the person""s ear to permit the person to speak the acoustic input signal into the microphone and to listen to the acoustic output signal generated by the earpiece. When a communication device permits a person to speak and listen to another party at the same time this is known as full duplex voice communications. The keypad permits a person to enter data, such as a phone number, into the communication device. The volume level of the acoustic output signal from the earpiece is set at a high enough level so that the person""s ear can hear the level of the acoustic output signal generated by the earpiece, but not so high as to damage the person""s hearing.
As advances in technology have permitted communication devices to be made smaller in size and lighter in weight, the available space for implementing necessary features such as microphones and earpieces becomes limited. One such trend in miniaturization is evident when viewing the following United States patents: U.S. Pat. No. Des. 424,557, issued May 9, 2000 to Nagle et al. and titled xe2x80x9cCellular Telephone Housingxe2x80x9d; U.S. Pat. No. Des. 421,005, issued Feb. 22, 2000 to Lucaci et al. and titled xe2x80x9cTelephonexe2x80x9d; U.S. Pat. No. Des. 417,449 issued Dec. 7, 1999 to Harris et al. and titled xe2x80x9cPortable Telephonexe2x80x9d; all of which are assigned to the assignee of the present invention, and all of which are incorporated by reference herein. Each of these telephone designs has an earpiece, a display, a keypad and a microphone carried by one or more housings and arranged on the one housing or on two housings when the two housings are in their opened position during use. The earpiece and the microphone are located at opposite ends of the one or more housings during use to maximize the distance between the earpiece and the microphone and to align the earpiece and the microphone with a person""s ear and mouth, respectively, while minimizing the total length of the one or more housings. However, the advantage of having easily accessible features, such as a large keypad or large display, contrasts with the availability of limited space on the one or more housings of the telephone set to place such features.
One drawback to traditional communication devices is that the experienced volume level and frequency spectrum of the acoustic output signal varies as the acoustical impedance loading of the communication device to a person""s ear varies. The acoustical impedance loading is related to the volume of space, as well as any leakage paths, between the communication device and the listener""s ear. As the coupling of the communication device to the ear changes, the impedance loading changes. For example, FIG. 1 illustrates the frequency response of a conventional portable telephone prior to auditory enhancements. The graph in FIG. 1 illustrates the frequency response of three simulated acoustical loadings of the listener""s ear: a sealed ear simulator, a low leak simulator, and a high leak simulator. As illustrated, the frequency response varies greatly between the three loading situations, causing the listener to experience an undesirable acoustic response variability. This undesirable acoustic response variability can lead to such things as acoustic discomfort and unintelligibility of the communication during any given voice conversation as the communication device is moved from tightly sealed to the ear, to slightly removed from the ear (low leak), to further removed from the ear (high leakxe2x80x94i.e. xc2xc inch to xe2x85x9 inch off the ear).
Recently communication device designers have implemented mechanical equalization to improve the auditory experience of the listener and minimize the effects of changes in acoustical loading between the earpiece of the communication device to the listener""s ear. FIG. 2 illustrates the frequency response of a conventional portable telephone with the earpiece and associated mechanics optimized for a consistent acoustical response. The graph in FIG. 2 illustrates the frequency response of the same three simulated acoustical loadings of the listener""s ear illustrated and described in FIG. 1. As illustrated in FIG. 2, the lower frequencies are attenuated significantly under high leak acoustical loading situations versus sealed ear acoustical loading situations to as much as 25 dB at certain frequencies. Contrasted to the attenuation at low frequencies, the higher frequency responses merge toward a substantially similar level for the three simulated loadings. Although maintaining intelligibility across the frequency spectrum for all three loading situations, the result continues to be acoustically inconsistent voice communication for the listener as the acoustical loading of the listener""s ear varies.
Therefore, what is needed is a low cost, minimal sized means for providing a constant frequency response and volume level to a person using a communication device independent of the unique characteristics of the user""s ear and how well the communication device is acoustically coupled to the user""s ear.